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Humphreys, Anna M.

Mechanical Engineering

2021

M.S.

With the popularity of small satellites on the rise in the aerospace industry, the need for propulsion systems developed specifically for small satellite applications is ever-present. An emerging subsection of the small satellite field is CubeSats, which are cubic satellites measuring 10cm per side and weighing less than 1.33kg. At launch, CubeSats are fixed to a larger rocket but are deployed once in orbit and must rely on their own propulsion system for orbit control and maneuverability. While propulsion systems exist for rockets and satellites that are orders of magnitude larger, these traditional systems are often too bulky and complex to simply be scaled down to a size conducive to use on CubeSats. Benchmark Space Systems, located in Burlington, VT, has developed a cutting-edge On-Demand Pressurization System, or ODPS, which uses a solid powder propellant known as Azodicarbonamide (Azo) to pressurize a fuel tank once in orbit. At high temperatures, around 230oC, this powder decomposes in an exothermic reaction, producing a large volume of gas contained within a small chemical storage tank. Upon release of this gas, the entire system is pressurized to provide thrust for the small satellite. The following thesis will outline the motivation, execution and results of tests performed at Benchmark Space Systems to quantify the pressure and temperature spikes resulting from the heating and subsequent decomposition of varying amounts of Azo. This research involved the development of an experimental apparatus that would simulate a controlled Azo decomposition event in space, while capturing the pressure and temperature effects using a combination of piezoelectric pressure sensors and thermocouples. The first round of testing, conducted in a blast chamber under atmospheric pressure, sought to understand how the magnitude of the pressure spike is related to the amount of propellant that is decomposed. It was found that the pressure and temperature spikes behaved as blast waves, and the data established a linear relationship between amount of propellant and shock wave overpressure magnitude. The second round of testing, conducted under vacuum, sought to understand the pressure and temperature spikes as they would behave in space, where the absence of a medium does not allow convective heat loss through the surrounding fluid. This round of testing aimed to visualize a phenomenon known as 'fratricide,' which occurs when there is more than one loaded propellant tank present in the system, and the decomposition effects of one reaction incite an unwanted, secondary reaction downstream. Ultimately, these tests were successful in quantifying and visualizing fratricide, and clearly showed multiple pressure and temperature shocks in setups where more than one tank was loaded. They also showed a clear relationship between amount of propellant and shock overpressure magnitude as well as system equilibrium pressure. Understanding the behavior of the resulting gas will allow for lighter and more predictable propulsion system configurations.